Conventionally, an electronic instrument includes a power conversion circuit for converting frequency or voltage of any input power source into those of a power source for various devices in the electronic instrument. In particular, a switching method is widely used for size reduction and higher efficiency. However, in the power source circuit using the switching method, a switching operation generates a high frequency switching noise, and the noise causes terminal noise or unwanted electromagnetic waves.
As a noise filter that reduces such unwanted electromagnetic waves, PTL 1 discloses a line bypass capacitor (hereinafter referred to as a “Y capacitor”). FIG. 7 illustrates an example of a filter circuit used in a conventional switching power source circuit. The filter circuit 100 illustrated in FIG. 7 includes Y capacitors 104 and 105 connected between a power supply line of each of a pair of power supply lines 101 and 102 and the ground (hereinafter referred to as “GND”) 103. The Y capacitors 104 and 105 remove common mode components of a high frequency switching noise generated from a switching element onto the GND 103, and reduce terminal noise or unwanted electromagnetic waves.
Two power supply lines connected from an input power source to the GND generally have the same impedance so that the two power supply lines are in balance with the GND. For example, in the filter circuit 100 illustrated in FIG. 7, the two Y capacitors 104 and 105 have the same capacitance value to keep balance.
However, in order to prevent failure of a circuit element, a switching power source includes an excess current protection element (for example, fuse) on an input side of a Y capacitor on one of two power supply lines connected from an input power source. The excess current protection element has a higher inductance than circuit wiring. Thus, if a switching frequency increases with decreasing size and increasing efficiency of the switching power source, an impedance by an inductance of the excess current protection element increases to increase imbalance between the two power supply lines. If there is imbalance between the two power supply lines, a normal mode component of the high frequency noise flowing through the two power supply lines is converted into a common mode component, thereby increasing common mode noise.